1. Field of the Invention
This invention relates to a key telephone system, in which a plurality of key telephone sets are selectively connected to a plurality of CO lines by the use of a plurality of talking lines under control of a key service unit and a plurality of control lines.
2. Description of the Prior Art
A conventional key telephone system has employed a cable with a large number of conductors for interconnecting a key service unit and key telephone sets. However, the system lately shows a tendency to become large-sized and multifunctional, and the costs of materials and construction are increasing, which inevitably raises the cost of the system. Accordingly, the reduction of the number of cable conductors is an effective solution for the problem of cutting costs. For decreasing the number of cable conductors, it is the practice in the art that the function of switching a CO line or a like communication line, which is achieved in the key telephone set at present, is transferred to the key service unit, as is usually seen in an exchange or the like. However, this heretofore employed method has the defect that the key service unit becomes bulky and expensive.
As regards the CO line holding operation, there has heretofore been employed means for transmitting CO line holding information by time division multiplexing between the key service unit and the key telephone set. With this method, however, holding information of one time slot is provided for each CO line, so that many time slots are needed in connection with means for transmitting the abovesaid information from the key telephone set to the key service unit. This results in raising a transmission frequency, which becomes a serious obstacle with an increase in the number of CO lines used.
Further, a privacy device of a key telephone system, heretofore employed, is packaged in the key telephone set and requires a relay contact for cutting off the communication line so as to achieve the privacy operation. This is disadvantageous for the package of the key telephone set and is an obstacle in the quality, performance, cost and maintenance of the key telephone set.
Moreover, there has heretofore been employed a method of transmitting a control signal between the key service unit and the key telephone set by the time division multiplex system, but this method requires a signal source (clock or the like) which is synchronized with both the key service unit and the key telephone set. Heretofore, there has been proposed a method in which one signal source is provided in the key service unit and a signal is transmitted therefrom to the key telephone set through a private channel, or a method in which a signal super-imposed on data is sent out to a data channel from the key service unit and only clock pulses are separated from the signal in the key telephone set. But the former requires a special cable conductor and the latter introduces complexity in the key telephone system.